Spread spectrum clock generator capable of frequency modulation with high accuracy

Abstract

In a spread spectrum clock generator, a DLL circuit delays an oscillation clock signal from a VCO and outputs ten delay clock signals having different phases respectively. A selector selects any one of the ten delay clock signals, and outputs a selected clock signal. A control circuit controls a signal selection operation of the selector. A feedback frequency divider divides a frequency of the selected clock signal by a frequency division ratio N, and generates a comparison clock signal. In this manner, a phase of the comparison clock signal can be fine-tuned. Therefore, a spread spectrum clock generator capable of frequency modulation with high accuracy can be obtained.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to a clock generator, and more particularly to a spread spectrum clock generator.[0003]2. Description of the Background Art[0004]A spread spectrum clock generator (SSCG) modulates a frequency of an oscillation clock signal so as to spread a bandwidth of a clock signal. Accordingly, EMI (Electro Magnetic Interference) noise is lowered.[0005]Some of conventional spread spectrum clock generators provided with a PLL (Phase Locked Loop) circuit include an input frequency divider dividing a frequency of an external clock signal and providing a reference clock signal to the PLL circuit, a feedback frequency divider dividing a frequency of an oscillation clock signal from an oscillator in the PLL circuit for feedback, and a control circuit modifying and controlling a frequency division ratio of the input frequency divider and the feedback frequency divider.[0006]For example, U.S. Pat. No. 6,377,646...

AI Technical Summary

Benefits of technology

[0009]Accordingly, a primary object of the present invention is to provide a spread spectrum clock generator capable of frequency modulation with high accuracy.

[0010]A clock generator according to the present invention includes an internal clock generator generating an oscillation clock signal obtained by multiplying a frequency of a reference clock signal, in synchronization with the received reference clock signal. The internal clock generator includes: a phase comparator circuit comparing phases of the reference clock signal and an internally generated comparison clock signal and outputting a phase difference signal in accordance with a comparison result; an oscillation circuit generating the oscillation clock signal based on the phase difference signal; a delay circuit delaying the oscillation clock signal so as to generate a plurality of delay clock signals having different phases respectively; a selection circuit selecting and outputting any one of the plurality of delay clock signals; and a frequency divider dividing a frequency of an output signal from the selection circuit by a predetermined frequency division ratio so as to generate the comparison clock signal. Thus, the phase of the oscillation clock signal can finely be tuned. Therefore, a spread spectrum clock generator capable of frequency modulation with high accuracy can be implemented.

[0011]Another clock generator according to the present invention includes: a delay circuit delaying a received clock signal so as to generate a plurality of delay clock signals having different phases respectively; a selection circuit selecting and outputting any one of the plurality of delay clock signals; a frequency divider dividing a frequency of an output signal from the selection circuit by a predetermined frequency division ratio so as to generate a reference clock signal; and an internal clock generator generating an oscillation clock signal obtained by multiplying a frequency of the reference clock signal, in synchronization with the reference clock signal. Here again, the phase of the oscillation clock signal can finely be tuned. Therefore, a spread spectrum clock generator capable of frequency modulation with high accuracy can be implemented.

Problems solved by technology

With such a method of controlling and modifying the frequency division ratio of the frequency divider, however, a frequency multiplication factor may be restricted by a value of the frequency division ratio.

Therefore, fine tuning of the frequency may be difficult depending on a condition, and accuracy in frequency modulation has been insufficient.

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